Resistive sheet goods generally include an organic film (polymer or paper) to which has been affixed a thin electrically conducting layer. Such materials can be applied to heating elements, resistive loads in flexible circuits, resistive terminations for antennas, and resistive components of composite materials for absorption of electromagnetic waves. In the latter category, these films are known as R-cards and are integral parts of Salisbury Screens, Jaumann absorbers and lossy Honeycomb. Controlling the surface resistance, measured in ohms per square (Ω/□) is critical for these applications both in terms of its precise value and its variation during the manufacturing process. Therefore, methods for the measurement of such properties are important both in a controlled laboratory environment as well as on the production floor. Precise determination of the surface resistance is the desired application in laboratory instruments, whereas relative determination of the same, say in comparison to a calibration standard, is frequently the critical application in a production environment.
In a laboratory environment, careful placement of the sheet may be an acceptable requirement for attaining high levels of accuracy and the time to perform the measurement is not necessarily critical. However, in a production environment the sheet goods may be moving as fast as 100 feet per minute past the sensor. Thus, it is clear that the details of the measurement set-up and the data reduction algorithms may differ depending on the application.
Depending on the manufacturing process and the organic substrate used, the sheet goods' surface resistance may be direction dependent. That is, the sheet resistance in the “down web” and “cross web” directions may differ. This anisotropy may be unintentional and as high as a ratio of 3 to 1, or it could be designed into the sheet by the use of imprinted geometric patterns and be as high as 10 to 1. The subsequent application of anisotropic sheets to a finished product such as a composite material may take advantage of this anisotropy to yield a highly anisotropic product or average it out to result in a net isotropic product. In the former case, accurate measurement of the anisotropy ratio in the sheets may be critical, whereas in the later case it is the average direction-independent resistance that matters. Thus, surface resistance measurement sensors able to measure both direction-specific and direction-averaged sheet resistance values are desired.
Although destructive measurement methods can be used, a non-destructive measurement method is generally desired. In principle, a small square sample of a resistive sheet could be cut, two opposing edges connected to electrodes and the resulting complex current measured under an applied alternating voltage. However, such an approach is slow, wasteful and highly undesirable for profitable manufacturing. Furthermore, many materials of interest are manufactured by processes that leave a disconnected network of conducting material on the film. Such a coating is strongly frequency dependent and may register as an open circuit at DC while appearing highly conductive at microwave frequencies. The prior art approach just described of measuring a small square sheet sample will not necessarily accurately represent the response of the sheet to electromagnetic waves in the microwave range. This is because the electric field in the test set-up described is essentially quasi-electrostatic by nature and thus easily distorted by other nearby high permittivity structures. To measure the frequency dependent conductance of a sheet in such a set-up would require guaranteeing that the electric flux density is always guided by the sheet from electrode to electrode. This is nearly impossible to guarantee.